The present invention relates to a device and a method for producing polybutylene succinate.
Polybutylene succinate is an aliphatic polyester produced by polycondensation of succinic acid or a derivative thereof with 1,4-butanediol.
A known method for synthesizing polybutylene succinate comprises performing an esterification reaction of succinic acid or a derivative thereof with 1,4-butanediol, and then heating and agitating the thus obtained ester in a high vacuum environment for a transesterification reaction (polycondensation), for example. An esterification reaction is carried out as follows. A carboxyl group of succinic acid or a terminal group of a derivative thereof is caused to undergo an esterification reaction with hydroxyl groups of 1,4-butanediol at a given temperature and pressure, so as to generate an oligomer having hydroxyl groups at its ends. In a polycondensation step, a transesterification reaction is carried out in the presence of a catalyst and vacuum devolatilization of 1,4-butanediol, which is a by-product thereof, is carried out, so as to increase molecular weight and viscosity. Therefore, a process is required that allows the system to be maintained at a high temperature and satisfies vacuum conditions by which the degree of vacuum is increased as the process proceeds is required. At this time, mechanical agitation is carried out to distill off elimination components existing in the surface layer of the reactant, so as to sufficiently increase vaporization surface area for by-products and the rate (surface update rate) of transfer of by-products from within the reaction solution to the vaporization surface.
In a polycondensation step, ester is heated and agitated in a vacuum environment, so that the polymer molecular chain grows due to a transesterification reaction. At this time, 1,4-butanediol as a by-product is removed by devolatilization because of reduced pressure, so that the reaction proceeds. Accordingly, in general, the degree of vacuum gradually increases as the reaction proceeds. In the case of polybutylene succinate, polymer molecules may be subjected to crosslinking in view of increasing polymer elasticity (elastomer formation). Two types of crosslinking method are: a method that involves mixing, in advance, succinic acid with a substance having a molecular size similar to that of succinic acid and having a factor capable of crosslinking, in addition to two carboxyl groups; and a method that involves mixing a coupling agent (e.g., urethane bond formation with the use of isocyanates) that reacts with terminal hydroxyl groups of a plurality of polymers after polymerization.
Examples of such a substance used for the former method include the one a hydroxyl group as a crosslinking factor and undergoing side-chain growth by polycondensation reaction or binding to other polymer molecules (i.e. malic acid), and the one having a double bond between carbon atoms as a crosslinking factor and crosslinking polymer molecules by conducting radical polymerization reaction after completion of polycondensation (i.e. itaconic acid). The use of malic acid is problematic in that molecule cyclization takes place during side-chain growth so that a terminal group serving as a growth point for polymerization reaction is lost, and thus the growth of a polymer molecular chain (that is, molecular weight increase) may be inhibited. Hence, the discovery of polymerization process conditions has been required to avoid this result.
A known method for coping with such a problem is the method disclosed in Patent Document 1 that involves adjusting polycondensation process conditions. According to the method disclosed in Patent Document 1, a polymer having a target molecular weight can be efficiently produced. However, the conditions disclosed in Patent Document 1 are problematic in that it is difficult to vary polycondensation process conditions depending on the progress of the reaction, and thus molecule cyclization may take place and a molecular weight increase may halt.
Meanwhile, a known method by which polycondensation process conditions can be varied depending on the progress of the reaction is a method for producing polybutyrene terephthalate, which is disclosed in Patent Document 2. According to the method disclosed in Patent Document 2, polycondensation process conditions can be varied depending on reaction progress. However, polycondensation of polybutyrene terephthalate does not cause molecular weight increase to be inhibited. Hence, a desired molecular weight may not be obtained if the process conditions are applied to the polycondensation of polybutylene succinate.    Patent Document 1: JP Patent No. 2713108    Patent Document 2: JP Patent No. 3847765